Inducing strain in the channels of metal gate transistors

ABSTRACT

In a metal gate replacement process, strain may be selectively induced in the channels of NMOS and PMOS transistors. For example, a material having a higher coefficient of thermal expansion than the substrate may be used to form the gate electrodes of PMOS transistors. A material with a lower coefficient of thermal expansion than that of the substrate may be used to form the gate electrodes of NMOS transistors.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/953,295, filed on Sep. 29, 2004 now U.S. Pat. No. 7,902,058.

BACKGROUND

This invention relates generally to the fabrication of integratedcircuits.

When making a complementary metal oxide semiconductor (CMOS) device thatincludes metal gate electrodes, it may be necessary to make the NMOS andPMOS gate electrodes from different materials. A replacement gateprocess may be used to form gate electrodes from different metals. Inthat process, a first polysilicon layer, bracketed by a pair of spacers,is removed to create a trench between the spacers. The trench is filledwith a first metal. The second polysilicon layer is then removed, andreplaced with a second metal that differs from the first metal.

To increase performance of NMOS and PMOS deep sub-micron transistors inCMOS technology, current state-of-the-art technology uses compressivestress in the channel of the PMOS transistors, and tensile stress in thecase of NMOS transistors. This is usually achieved by substrate inducedstrain which is a very expensive technology option and also difficult toimplement using a single substrate approach.

Thus, there is a need for a way to improve the performance of metal gatefield effect transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1N represent enlarged, cross-sections of structures that may beformed when carrying out an embodiment of the method of the presentinvention.

Features shown in these Figures are not intended to be drawn to scale.

DETAILED DESCRIPTION

A semiconductor structure includes first part 101 and second part 102 ofsubstrate 100 as shown in FIG. 1A. Isolation region 103 separates firstpart 101 from second part 102. First sacrificial layer 104 is formed onfirst dummy dielectric layer 105, and second sacrificial layer 106 isformed on second dummy dielectric layer 107. Hard masks 130, 131 areformed on sacrificial layers 104, 106, and etch stop layers 132, 133 areformed on hard masks 130, 131.

Substrate 100 may comprise a bulk silicon or silicon-on-insulatorsubstructure. Alternatively, substrate 100 may comprise othermaterials—which may or may not be combined with silicon—such as:germanium, indium antimonide, lead telluride, indium arsenide, indiumphosphide, gallium arsenide, or gallium antimonide. Although a fewexamples of materials from which substrate 100 may be formed aredescribed here, any material that may serve as a foundation upon which asemiconductor device may be built falls within the spirit and scope ofthe present invention. Isolation region 103 may comprise silicondioxide, or other materials that may separate the transistor's activeregions.

First dummy dielectric layer 105 and second dummy dielectric layer 107may each comprise silicon dioxide, or other materials that may protectthe substrate—e.g., carbon doped silicon dioxide, silicon oxynitride,silicon nitride, or a nitrided silicon dioxide. Dummy dielectric layers105, 107 may, for example, be at least about 10 Angstroms thick, andbetween about 15 Angstroms and about 30 Angstroms thick in oneembodiment. Dummy dielectric layers 105, 107 may comprise a highquality, dense thermally grown silicon dioxide layer. Such a layer maybe between about 20 and about 30 Angstroms thick in one embodiment.

Dummy dielectric layers 105, 107 may instead comprise a nitrided silicondioxide, e.g., a dielectric layer formed by applying a high temperatureanneal to a very thin silicon dioxide layer in the presence of nitrogen,or by striking a nitrogen plasma in the presence of such a silicondioxide layer. In one embodiment, such an anneal takes place at about600° C. for about 30 seconds. Annealing such a silicon dioxide layer ina nitrogen ambient may cause nitrogen to bond to that layer's surface,which may yield a more robust protective layer. When dummy dielectriclayers 105, 107 comprise a nitrided silicon dioxide, they may, forexample, be between about 10 and about 30 Angstroms thick and betweenabout 15 and about 30 Angstroms thick in one embodiment.

Sacrificial layers 104, 106 may comprise polysilicon and may, forexample, be between about 100 and about 2,000 Angstroms thick andbetween about 500 and about 1,600 Angstroms thick in one embodiment.Hard masks 130, 131 may comprise silicon nitride and may, for example,be between about 100 and about 500 Angstroms thick and between about 200and about 350 Angstroms thick in one embodiment. Etch stop layers 132,133 may comprise a material that will be removed at a substantiallyslower rate than silicon nitride will be removed when an appropriateetch process is applied. Etch stop layers 132, 133 may, for example, bemade from an oxide (e.g., silicon dioxide or a metal oxide such ashafnium dioxide), a carbide (e.g., silicon carbide or a metal carbide),a carbon doped silicon oxide, or a carbon doped silicon nitride. Etchstop layers 132, 133 may, for example, be between about 200 and about1,200 Angstroms thick and may be between about 400 and about 600Angstroms thick in one embodiment.

When sacrificial layers 104, 106 comprise polysilicon, and hard masklayers 130, 131 comprise silicon nitride, the FIG. 1A structure may bemade in the following way. A dummy dielectric layer, which may comprisesilicon dioxide, is formed on substrate 100 (e.g., via a conventionalthermal growth process), followed by forming a polysilicon layer on thedielectric layer (e.g., via a conventional deposition process). Usingconventional deposition techniques, a silicon nitride layer is formed onthe polysilicon layer, and an etch stop layer is formed on the siliconnitride layer. The etch stop, silicon nitride, polysilicon, and dummydielectric layers are then patterned to form patterned etch stop layers132, 133, patterned silicon nitride layers 130, 131, patternedpolysilicon layers 104, 106, and patterned dummy dielectric layers 105,107. When the dummy dielectric layer comprises silicon dioxide, one mayapply routine etch processes to pattern the polysilicon and dummydielectric layers.

After forming the FIG. 1A structure, spacers may be formed on oppositesides of sacrificial layers 104, 106. When those spacers comprisesilicon nitride, they may be formed in the following way. First, asilicon nitride layer 134 of substantially uniform thickness, forexample, less than about 1000 Angstroms thick, is deposited over theentire structure, producing the structure shown in FIG. 1B. Conventionaldeposition processes may be used to generate that structure.

In one embodiment, silicon nitride layer 134 may be deposited directlyon substrate 100, patterned etch stop layers 132, 133, and oppositesides of sacrificial layers 104, 106—without first forming a bufferoxide layer on substrate 100 and layers 104, 106. In other embodiments,however, such a buffer oxide layer may be formed prior to forming layer134. Similarly, although not shown in FIG. 1B, a second oxide may beformed on layer 134 prior to etching that layer. If used, such an oxidemay enable the subsequent silicon nitride etch step to generate anL-shaped spacer.

Silicon nitride layer 134 may be etched using a conventional process foranisotropically etching silicon nitride to create the sidewall spacers108, 109, 110, and 111 shown in FIG. 1C. Etch stop layers 132, 133prevent such an anisotropic etch step from removing hard masks 130, 131,when silicon nitride layer 134 is etched—even when hard masks 130, 131comprise silicon nitride. As a result of that etch step, sacrificiallayer 104 is bracketed by a pair of sidewall spacers 108, 109, andsacrificial layer 106 is bracketed by a pair of sidewall spacers 110,111.

As is typically done, it may be desirable to perform multiple maskingand ion implantation steps to create lightly implanted regions 135 a-138a near layers 104, 106 (that will ultimately serve as tip regions forthe device's source and drain regions), prior to forming spacers 108,109, 110, 111 on sacrificial layers 104, 106 as shown in FIG. 1D. Alsoas is typically done, the source and drain regions 135-138 may beformed, after forming spacers 108, 109, 110, 111, by implanting ionsinto parts 101 and 102 of substrate 100, followed by applying anappropriate anneal step.

When sacrificial layers 104, 106 comprise polysilicon, an ionimplantation and anneal sequence used to form n-type source and drainregions within part 101 of substrate 100 may dope polysilicon layer 104n-type at the same time. Similarly, an ion implantation and annealsequence used to form p-type source and drain regions within part 102 ofsubstrate 100 may dope polysilicon layer 106 p-type. When dopingpolysilicon layer 106 with boron, that layer may include that element ata sufficient concentration to ensure that a subsequent wet etch process,for removing n-type polysilicon layer 104, will not remove a significantamount of p-type polysilicon layer 106.

If dummy dielectric layers 105, 107 are at least about 20 Angstromsthick—when made of silicon dioxide—and at least about 10 Angstromsthick—when made from a nitrided silicon dioxide, they may prevent asignificant number of ions from penetrating through layers 104, 106 andlayers 105, 107. For that reason, replacing a relatively thin silicondioxide layer with a relatively thick dummy dielectric layer may enableone to optimize the process used to implant ions into the source anddrain regions without having to consider whether that process will drivetoo many ions into the channel. After the ion implantation and annealsteps, part of the source and drain regions may be converted to asuicide using well known process steps.

After forming spacers 108, 109, 110, 111, dielectric layer 112 may bedeposited over the device, generating the FIG. 1D structure. Dielectriclayer 112 may comprise silicon dioxide, or a low-k material. Dielectriclayer 112 may be doped with phosphorus, boron, or other elements, andmay be formed using a high density plasma deposition process. By thisstage of the process, source and drain regions 135, 136, 137, 138, whichare capped by silicided regions 139, 140, 141, 142, have already beenformed. Conventional process steps, materials, and equipment may be usedto generate the structures represented by FIGS. 1A-1D, as will beapparent to those skilled in the art. Those structures may include otherfeatures—not shown, so as not to obscure the method of the presentinvention—that may be formed using conventional process steps.

Dielectric layer 112 is removed from patterned etch stop layers 132,133, which are, in turn, removed from hard masks 130, 131, which are, inturn, removed from patterned sacrificial layers 104, 106, producing theFIG. 1E structure. A conventional chemical mechanical polishing (“CMP”)operation may be applied to remove that part of dielectric layer 112,patterned etch stop layers 132, 133, and hard masks 130, 131. Etch stoplayers 132, 133 and hard masks 130, 131 must be removed to exposepatterned sacrificial layers 104, 106. Etch stop layers 132, 133 andhard masks 130, 131 may be polished from the surface of layers 104, 106,when dielectric layer 112 is polished—as they will have served theirpurpose by that stage in the process.

After forming the FIG. 1E structure, sacrificial layer 104 is removed togenerate trench 113 that is positioned between sidewall spacers 108,109—producing the structure shown in FIG. 1F. In one embodiment, a wetetch process that is selective for layer 104 over sacrificial layer 106is applied to remove layer 104 without removing significant portions oflayer 106.

When sacrificial layer 104 is doped n-type, and sacrificial layer 106 isdoped p-type (e.g., with boron), such a wet etch process may compriseexposing sacrificial layer 104 to an aqueous solution that comprises asource of hydroxide for a sufficient time at a sufficient temperature toremove substantially all of layer 104. That source of hydroxide maycomprise between about 2 and about 30 percent ammonium hydroxide or atetraalkyl ammonium hydroxide, e.g., tetramethyl ammonium hydroxide(“TMAH”), by volume in deionized water.

Sacrificial layer 104 may be selectively removed by exposing it to asolution, which is maintained at a temperature between about 15° C. andabout 90° C. (and preferably below about 40° C.), that comprises betweenabout 2 and about 30 percent ammonium hydroxide by volume in deionizedwater. During that exposure step, which may last at least one minute, itmay be desirable to apply sonic energy at a frequency of between about10 KHz and about 2,000 KHz, while dissipating at between about 1 andabout 10 Watts/cm².

Sacrificial layer 104, for example, with a thickness of about 1,350Angstroms, may be selectively removed by exposing it at about 25° C. forabout 30 minutes to a solution that comprises about 15 percent ammoniumhydroxide by volume in deionized water, while applying sonic energy atabout 1,000 KHz—dissipating at about 5 Watts/cm². Such an etch processshould remove substantially all of an n-type polysilicon layer withoutremoving a meaningful amount of a p-type polysilicon layer.

As an alternative, sacrificial layer 104 may be selectively removed byexposing it for at least one minute to a solution, which is maintainedat a temperature between about 60° C. and about 90° C., that comprisesbetween about 20 and about 30 percent TMAH by volume in deionized water,while applying sonic energy. Removing sacrificial gate electrode layer104, with a thickness of about 1,350 Angstroms, by exposing it at about80° C. for about 2 minutes to a solution that comprises about 25 percentTMAH by volume in deionized water, while applying sonic energy at about1,000 KHz—dissipating at about 5 watts/cm²—may remove substantially allof layer 104 without removing a significant amount of layer 106. Firstdummy dielectric layer 105 may be sufficiently thick to prevent theetchant that is applied to remove sacrificial layer 104 from reachingthe channel region that is located beneath first dummy dielectric layer105.

After removing sacrificial layer 104, first dummy dielectric layer 105is removed. When first dummy dielectric layer 105 comprises silicondioxide, it may be removed using an etch process that is selective forsilicon dioxide to generate the FIG. 1G structure. Such etch processesinclude: exposing layer 105 to a solution that includes about 1 percenthydrofluoric acid (HF) in deionized water, or applying a dry etchprocess that employs a fluorocarbon based plasma. Layer 105 may beexposed for a limited time, as the etch process for removing layer 105may also remove part of dielectric layer 112. With that in mind, if a 1percent HF based solution is used to remove layer 105, the device may beexposed to that solution for less than about 60 seconds, for example forabout 30 seconds or less. It may be possible to remove layer 105 withoutremoving a significant amount of dielectric layer 112, if layer 105 isless than about 30 angstroms thick, when initially deposited.

After removing first dummy dielectric layer 105, gate dielectric layer114 is formed on substrate 100 at the bottom of trench 113, generatingthe FIG. 1H structure. Although gate dielectric layer 114 may compriseany material that may serve as a gate dielectric for an NMOS transistorthat includes a metal gate electrode, gate dielectric layer 114 maycomprise a high-k metal oxide dielectric material. Some of the materialsthat may be used to make high-k gate dielectric 114 include: hafniumoxide, hafnium silicon oxide, lanthanum oxide, zirconium oxide,zirconium silicon oxide, tantalum oxide, titanium oxide, bariumstrontium titanium oxide, barium titanium oxide, strontium titaniumoxide, yttrium oxide, aluminum oxide, lead scandium tantalum oxide, andlead zinc niobate. Particularly useful metal oxides include hafniumoxide, zirconium oxide, and aluminum oxide. Although a few examples ofmetal oxides that may be used to form high-k gate dielectric layer 114are described here, that layer may be made from other metal oxides aswell.

High-k gate dielectric layer 114 may be formed on substrate 100 using aconventional deposition method, e.g., a conventional chemical vapordeposition (“CVD”), low pressure CVD, or physical vapor deposition(“PVD”) process. Preferably, a conventional atomic layer CVD process isused. In such a process, a metal oxide precursor (e.g., a metalchloride) and steam may be fed at selected flow rates into a CVDreactor, which is then operated at a selected temperature and pressureto generate an atomically smooth interface between substrate 100 andhigh-k gate dielectric layer 114. The CVD reactor should be operatedlong enough to form a layer with the desired thickness. In mostapplications, high-k gate dielectric layer 114 may, for example, be lessthan about 60 Angstroms thick and, in one embodiment, between about 5Angstroms and about 40 Angstroms thick.

As shown in FIG. 1H, when an atomic layer CVD process is used to formhigh-k gate dielectric layer 114, that layer will form on the verticalsides of trench 113 in addition to forming on the bottom of that trench.If high-k gate dielectric layer 114 comprises an oxide, it may manifestoxygen vacancies at random surface sites and unacceptable impuritylevels, depending upon the process used to make it. It may be desirableto remove impurities from layer 114, and to oxidize it to generate alayer with a nearly idealized metal:oxygen stoichiometry, after layer114 is deposited.

To remove impurities from that layer and to increase that layer's oxygencontent, a wet chemical treatment may be applied to high-k gatedielectric layer 114. Such a wet chemical treatment may compriseexposing high-k gate dielectric layer 114 to a solution that compriseshydrogen peroxide at a sufficient temperature for a sufficient time toremove impurities from high-k gate dielectric layer 114 and to increasethe oxygen content of high-k gate dielectric layer 114. The appropriatetime and temperature at which high-k gate dielectric layer 114 isexposed may depend upon the desired thickness and other properties forhigh-k gate dielectric layer 114.

When high-k gate dielectric layer 114 is exposed to a hydrogen peroxidebased solution, an aqueous solution that contains between about 2% andabout 30% hydrogen peroxide by volume may be used. That exposure stepshould take place at between about 15° C. and about 40° C. for at leastabout one minute. In a particularly preferred embodiment, high-k gatedielectric layer 114 is exposed to an aqueous solution that containsabout 6.7% H₂O₂ by volume for about 10 minutes at a temperature of about25° C. During that exposure step, it may be desirable to apply sonicenergy at a frequency of between about 10 KHz and about 2,000 KHz, whiledissipating at between about 1 and about 10 Watts/cm². In oneembodiment, sonic energy may be applied at a frequency of about 1,000KHz, while dissipating at about 5 Watts/cm².

Although not shown in FIG. 1H, it may be desirable to form a cappinglayer, which is no more than about five monolayers thick, on high-k gatedielectric layer 114. Such a capping layer may be formed by sputteringone to five monolayers of silicon, or another material, onto the surfaceof high-k gate dielectric layer 114. The capping layer may then beoxidized, e.g., by using a plasma enhanced chemical vapor depositionprocess or a solution that contains an oxidizing agent, to form acapping dielectric oxide.

Although in some embodiments it may be desirable to form a capping layeron gate dielectric layer 114, in the illustrated embodiment, n-typemetal layer 115 is formed directly on layer 114 to fill trench 113 andto generate the FIG. 11 structure. N-type metal layer 115 may compriseany n-type conductive material from which a metal NMOS gate electrodemay be derived. N-type metal layer 115 preferably has thermal stabilitycharacteristics that render it suitable for making a metal NMOS gateelectrode for a semiconductor device.

Materials that may be used to form n-type metal layer 115 include:hafnium, zirconium, titanium, tantalum, aluminum, and their alloys,e.g., metal carbides that include these elements, i.e., hafnium carbide,zirconium carbide, titanium carbide, tantalum carbide, and aluminumcarbide. The metal used to form the layer may be the same or a differentmetal than the metal component of the metal oxide dielectric layer 114.N-type metal layer 115 may be formed on high-k gate dielectric layer 114using well known PVD or CVD processes, e.g., conventional sputter oratomic layer CVD processes. As shown in FIG. 1J, n-type metal layer 115is removed except where it fills trench 113. Layer 115 may be removedfrom other portions of the device via a wet or dry etch process, or anappropriate CMP operation. Dielectric 112 may serve as an etch or polishstop, when layer 115 is removed from its surface.

N-type metal layer 115 may serve as a metal NMOS gate electrode that hasa workfunction that is between about 3.9 eV and about 4.2 eV, and thatmay, for example, be between about 100 Angstroms and about 2,000Angstroms thick and, in one embodiment, is between about 500 Angstromsand about 1,600 Angstroms thick. FIGS. 1I and 1J represent structures inwhich n-type metal layer 115 fills the portion of trench 113 over ann-type metal layer 202. The n-type metal layer 202 serves as the workfunction metal. The layer 202 may, for example, be between about 50 andabout 1,000 Angstroms thick, and, in one embodiment, at least about 100Angstroms thick.

The layer 115 may have a lower coefficient of thermal expansion than thesubstrate 100 which may be silicon. As a result, the layer 115 mayimpart tensile strain to the channel. With a silicon substrate, thelayer 202 may be formed of titanium carbide.

In embodiments in which trench 113 includes both a workfunction metallayer 202 and a trench fill metal 115, the resulting metal NMOS gateelectrode may be considered to comprise the combination of both theworkfunction metal and the trench fill metal. If a trench fill metal isdeposited on a workfunction metal, the trench fill metal may cover theentire device when deposited, forming a structure like the FIG. 1Istructure. That trench fill metal must then be polished back so that itfills only the trench, generating a structure like the FIG. 1Jstructure.

In the illustrated embodiment, after forming n-type metal layer 115within trench 113, sacrificial layer 106 is removed to generate trench150 that is positioned between sidewall spacers 110, 111—producing thestructure shown in FIG. 1K. In one embodiment, layer 106 is exposed to asolution that comprises between about 20 and about 30 percent TMAH byvolume in deionized water for a sufficient time at a sufficienttemperature (e.g., between about 60° C. and about 90° C.), whileapplying sonic energy, to remove all of layer 106 without removingsignificant portions of n-type metal layer 115.

Alternatively, a dry etch process may be applied to selectively removelayer 106. When sacrificial gate electrode layer 106 is doped p-type(e.g., with boron), such a dry etch process may comprise exposingsacrificial gate electrode layer 106 to a plasma derived from sulfurhexafluoride (“SF₆”), hydrogen bromide (“HBr”), hydrogen iodide (“HI”),chlorine, argon, and/or helium. Such a selective dry etch process maytake place in a parallel plate reactor or in an electron cyclotronresonance etcher.

Second dummy dielectric layer 107 may be removed and replaced with gatedielectric layer 160, using process steps like those identified above.Metal oxide dielectric layer 160 preferably comprises a high-k gatedielectric layer. Optionally, as mentioned above, a capping layer (whichmay be oxidized after it is deposited) may be formed on gate dielectriclayer 160 prior to filling trench 150 with a p-type metal.

In this embodiment, however, after replacing layer 107 with layer 160,the workfunction metal layer 20 is formed directly on layer 160. Then,the p-type metal layer 116 fills trench 150 to generate the FIG. 1Lstructure. P-type metal layer 116 may comprise any p-type conductivematerial from which a metal PMOS gate electrode may be derived and whichcompressively strains the channel to this end. The p-type metal layermay be one with a higher coefficient of thermal expansion than that ofthe substrate 100, which may be silicon. Examples of suitable metalsincludes boron carbide, tungsten, molybdenum, rhodium, vanadium,platinum, ruthenium, beryllium, palladium, cobalt, titanium, nickel,copper, tin, aluminum, lead, zinc, alloys, and suicides of thesematerials. In one embodiment, the use of a material having a coefficientof thermal expansion higher than that of tungsten (0.4×10⁻⁵ in./in./°C.) is advantageous. A relatively high deposition temperature, such as400° C., may be used in some embodiments, generating compressive strainin the channel and improving mobility. Because the fill material shrinksmore than the substrate, compressive strain is applied. P-type metallayer 116 preferably has thermal stability characteristics that renderit suitable for making a metal PMOS gate electrode for a semiconductordevice.

Materials that may be used to form p-type metal layer 116 include:ruthenium, palladium, platinum, cobalt, nickel, and conductive metaloxides, e.g., ruthenium oxide. The metal of the layer 116 may be thesame or different than the metal component of the metal oxide dielectriclayer 160. P-type metal layer 116 may be formed on gate dielectric layer160 using well known PVD or CVD processes, e.g., conventional sputter oratomic layer CVD processes. As shown in FIG. 1M, p-type metal layer 116is removed except where it fills trench 150. Layer 116 may be removedfrom other portions of the device via a wet or dry etch process, or anappropriate CMP operation, with dielectric 112 serving as an etch orpolish stop.

P-type metal layer 116 may serve as a metal PMOS gate electrode with aworkfunction that is between about 4.9 eV and about 5.2 eV, and thatmay, for example, be between about 100 Angstroms and about 2,000Angstroms thick and, in one embodiment, is between about 500 Angstromsand about 1,600 Angstroms thick.

Although FIGS. 1L and 1M represent structures in which p-type metallayer 116 fills all of trench 150, in alternative embodiments, p-typemetal layer 116 may fill only part of trench 150. As with the metal NMOSgate electrode, the remainder of the trench may be filled with amaterial that may be easily polished, e.g., tungsten, aluminum,titanium, or titanium nitride. In such an alternative embodiment, p-typemetal layer 116, which serves as the workfunction metal, may be betweenabout 50 and about 1,000 Angstroms thick. Like the metal NMOS gateelectrode, in embodiments in which trench 150 includes a workfunctionmetal and a trench fill metal, the resulting metal PMOS gate electrodemay be considered to comprise the combination of both the workfunctionmetal and the trench fill metal.

The vertical portions 114 a, 160 a of the gate dielectric 114, 160 donot significantly contribute to the performance of the resultingtransistor and would produce fringe capacitance. To this end, the FIG.1N structure may be exposed to a low angle ion implantion I as indicatedin FIG. 1N. The implantation I may implant silicon ions to convert thevertical portions 114 a, 160 a of the metal oxide dielectric 114, 160 toa ternary silicate 114 b, 160 b. The ternary silicate 114 b, 160 b has amuch lower dielectric constant. By exposing the upper surface of thesemiconductor structure to a silicon ion implant “I”, as indicated inFIG. 1N, the dielectric constant of the vertical portions 114 a, 160 aof the gate metal oxide dielectric 114, 160 may be reduced, therebyreducing the fringe capacitance that would otherwise have beencontributed by the unimplanted portions 114 a, 160.

In one embodiment, the implant angle may be from 30 to 60 degrees, thedose may be from le15 to le16 atoms per cm² and the energy may be 20 to30 key. The implantation may be repeated with an intervening 180° waferrotation, in some embodiments.

Selectively strained channels may be formed in both NMOS and PMOStransistors, taking advantage of the replacement gate process and usingdual metal types with the appropriate thermal expansion coefficients asfill metal for the gate trench process.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

What is claimed is:
 1. A semiconductor structure comprising: asubstrate; an NMOS gate structure including an NMOS workfunctionmaterial over said substrate and a first strain material over said NMOSworkfunction material having a sufficiently different coefficient ofthermal expansion than said substrate to strain said substrate whereinsaid NMOS workfunction material includes a first metal, said firststrain material includes a second metal; and a PMOS gate structureincluding a PMOS workfunction material over said substrate and a secondstrain material over said PMOS workfunction material having asufficiently different coefficient of thermal expansion than saidsubstrate to strain said substrate, said PMOS workfunction materialincludes a third metal, and said second strain material includes afourth metal, said first, second, third, and fourth metals beingdifferent metals.
 2. The structure of claim 1 wherein said second strainmaterial has a higher coefficient of thermal expansion than saidsubstrate.
 3. The structure of claim 1 wherein said first strainmaterial has a lower coefficient of thermal expansion than saidsubstrate.
 4. The structure of claim 1 wherein said second strainmaterial has a coefficient of thermal expansion greater than 0.4×10⁻⁵in./in.°/C.
 5. The structure of claim 1 wherein said first or secondstrain material includes a metal selected from the following group:zinc, lead, aluminum, tin, copper, nickel, titanium, cobalt, palladium,beryllium, molybdenum, ruthenium, platinum, vanadium, rhodium, tungsten,and boron.